He was a cousin of William the Conqueror, and supported the king during the 1088 revolt of Odo of Bayeux. After the revolt failed he was rewarded with great estates in Gloucestershire and elsewhere.
The chronology of his conquest of Glamorgan is uncertain.
After William the Conqueror's death Robert was a close adherant of his son William Rufus. Legend has it that Robert had ominous dreams in the days before Rufus' fatal hunting expedition, which postponed but did not prevent the outing. He was one of the first to gather in tears around Rufus' corpse, and he used his cloak to cover the late king's body on its journey to be buried in Winchester. How much of these stories are the invention of later days is unknown.
In any case Robert proved as loyal to Henry I as he had been to his predecessors, remaining on Henry's side in the several open conflicts with Henry's brother Robert Curthose. He severely injured in the head at the siege of Falaise in 1105; though he lived two more years he was never the same mentally.
Robert married Sibyl of Montgomery, by whom he probably only had one child, Mabel, who inherited his great estates and married Robert of Gloucester.